The present invention is directed to the preparation of bipolar membranes possessing both low electrical resistance and superior performance properties, durability, and high reproducibility. In particular, the invention relates to the preparation of the single film bipolar membranes involving, specifically, the surface treatment to remove the excess skin layer formed during the introduction of styrene and divinylbenzene on to the polymeric matrix system. Thereby the subsequent functionalization of the film with cationic groups and anionic groups may result in a substantially improved quality control and membrane uniformity.
Various ion exchange membranes, cationic and anionic, individually as well as laminae membranes, are well known in the art. Styrene-divinylbenzene copolymers with sulfonic acid ion exchange groups (cation-type) are fully disclosed, e.g. in U.S. Pat. No. 2,731,411. The anion-type, for example a styrene-divinylbenzene vinylpyridine membrane, is disclosed in U.S. Pat. No. 2,860,097. Cation and anion membranes based on polyethylenestyrene copolymers bonded together in a hydraulic press under heat and pressure to form two-ply membrane structures are also known as shown, for example in U.S. Pat. No. 3,372,101. Such membranes generally have the disadvantage of high electrical resistance, incurred during fusion; are prone to bubble or blister; and only operate at relatively low current densities, for short time periods, all of which render them unattractive for commercial electrodialysis operations.
Some single film bipolar membranes have also been disclosed. For example, some have been obtained by hydrolyzing one side and aminating the other side of a chlorosulfonated polyethylene sheet, as diclosed in U.S. Pat. No. 3,388,080. Membranes thus prepared, however, are relatively inefficient in that they have high voltage drops across them. Another single-film bipolar membrane, of the polyethylene-styrene divinylbenzene type, is disclosed in the Leitz U.S. Pat. No. 3,562,139. The latter membranes are designed specifically for desalination by electrodialysis, wherein, the direction of electrical current flow is periodically reversed. Such membranes behave asymmetrically transferring mainly cations when the cationic lamina of the membrane faces the cathode and transferring mainly anions when the anionic lamina faces the cathode. To the degree that any water splitting could be effected using the membrane described by Leitz in U.S. Pat. No. 3,562,139, the current efficiency of the contemplated desalination process would be decreased. Moreover, the membranes of U.S. Pat. No. 3,562,139 have only a relatively low level of cross-linking (approximately 0.5% active divinylbenzene) which results in relatively inferior ion selectivity. Also membranes of the kind disclosed in U.S. Pat. No. 3,562,139 possess design features, such as (a) a cation exchange group internal molality less than the anion exchange group internal molality, and (b) an anionic layer which is thinner than the cation layer, both of which aid the transport of the electrolyte through forbidden areas, i.e., against the Donnan exclusion forces, and possess only relatively low current efficiencies (50-70% desalination) at low electrolyte concentrations (0.03-0.06N) and current densities (8-25 amp.ft.sup.2).
Thus, while the preparation of low cross-linked, low ion-selective polyethylene-polystyrene bipolar membranes has been achieved, it is particularly difficult to obtain bipolar membranes with a relatively high number of cross-linking bonds, high functional group concentrations, high ion-selectivities, and yet have low membrane voltage drops and long operational capabilities at relatively high current densities and electrolyte concentrations. This invention discloses methods for obtaining single film bipolar membranes with these advantages and with excellent reproducibility and quality control.